«No cells selected» - no cell is selected. Mark the cells before running the operation.
«No plants selected» - no plant is selected.
«The selection contains an empty cell» - there is an empty cell among the selected ones, and the operation applies to plants only. Deselect it.
«Not enough free cells from the drop point: … needed, … available» - the destination box has fewer free cells than there are plants in the selection. The message names how many are needed and how many there are.
«The selection contains a plant that does not exist» - some of the selected plants have already been removed by another member. Refresh the map and select again.